An automatic chess compressor can be carried by an emergency worker and applied to a patient to stimulate blood circulation and breathing. Such a chest compressor usually includes a piston that moves up and down in a cylinder (assuming that the patient is reclined so his chest faces upward). The cylinder is held closely over the patient's chest as by a strap wrapped around the patient's chest area. The chest compressions may occur at a frequency such as ½ to one second apart, so the piston moves downward rapidly in each stroke.
A rapidly downwardly-moving piston tends to impart a downward pulse to the patient at the bottom of the stroke, which is not desirable. It is possible to use a spring that is fixed to the piston to reduce the downward force as the piston moves down. However, the spring force applied along most of the piston movement tends to slow piston movement so additional energy is required to move the piston. A spring has the advantage of more rapidly raising the piston after each chest compression, but it is found that resilience of the chest is sufficient to rapidly raise the piston after each downward stroke.
In most cases, the patient's chest should be compressed by about one to two inches in each stroke, to benefit blood circulation and breathing while avoiding harm to the patient's chest (e.g. by breaking a rib). The piston actually must apply a downward stroke of about two to four inches to produce a patient chest compression of about one to two inches because the backup such as the strap wrapped about the patient, presses into the patient and takes up some of the compression. For children and small adults, it is desirable to reduce the chest compression to near the minimum, while for adults of normal to large size it is desirable to use chest compressions close to the maximum. The chest compressor should be easily and rapidly convertible between different compression distances.